All together now!

For many years, silver chloride was the gold standard of photographic printing. The chemical’s high photosensitivity made basic imaging possible because it darkens when exposed to light. Now, the material that moved the world from sepia tones to classic black and white may presage the next evolutionary stage for science and technology.

Sound like hyperbole?

The goal of many researchers over the past few years has been to close the gap between real life and science fiction: Create nanoscopic materials that self-assemble into fully operating systems such as minuscule robots that sweep a room clean of allergens or that patrol your bloodstream looking for cancer cells. Aside from engineering issues involved in building automatons only a few nanometers across, there are a few essential design issues that are particularly perplexing. One of these concerns is communication – to perform any task, the individual machines in a cloud of nanorobots must be able to talk to each other.

At Pennsylvania State University, Ayusman Sen and his colleagues found that silver chloride particles have two key traits that will be the hallmark of nanorobots: They can propel themselves, and they can send messages that can be picked up by other AgCl particles. Both of these feats occur when the particles are exposed to ultraviolet light.

Sen’s group placed masses of AgCl particles in deionized water sitting in a well on a microscope slide, then shone UV light in the 320- to 380-nm range into the well through the objective. Low-intensity visible light was shone onto the sample as well for acquiring images of the process (see figures). After a few seconds, they saw the particles forming small clumps – the longer the UV light was on, the tighter the clumps became. Furthermore, the aggregations remained stable, with interparticle distances of 1 to 2 nm.

An aggregation of AgCl particles that had been irradiated previously with UV light were imaged with visible light (a) alone. After the UV light is switched back on (b-d), the aggregation loosens up; after it is switched off again, the group retightens (e-j). Scale bar = 20 μm, t = time in seconds.The clumping works, the team reported, because the AgCl particles lose ions when irradiated by UV rays. In a process called diffusiophoresis, other nearby particles sniff out the loose ions and move toward their source. Because this process is very similar to how living cells send and receive signals, inducing actions among both friendly and antagonistic neighboring cells, Sen says that the silver chloride system could be used as a mechanical model of intercellular communication.

In another experiment, Sen’s group alternated periods of UV light, leaving the visible light on continuously. Turning the UV off after about 10 minutes, they saw that the silver chloride particles aggregated more closely than before under the remaining visible light. When the UV was switched on again, the particles returned to their more loose aggregation state. Interestingly, this effect occurred only with AgCl particles that had undergone previous UV irradiation; fresh particles did not respond to visible light alone.

Under UV illumination (320 to 380 nm), silver chloride particles in deionized water accumulate into tight groups. Shown are particles with no UV (a), after 30 s of UV (b) and after 90 s of UV (c). Scale bar = 20 μm. Photos courtesy of Angewandte Chemie. Much research remains, but it is foreseeable that AgCl or similarly responsive particles could be used to carry antibodies or other “cargo” to cancer tumors, amyloid plaques or other troublesome cells and tissues.